The ability to separate a particle/fluid mixture into its separate components is desirable in many applications. Physical size exclusion filters can be used for this purpose, where the particles are trapped on the filter and the fluid flows through the filter. Examples of physical filters include those that operate by tangential flow filtration, depth flow filtration, hollow fiber filtration, and centrifugation. However, physical filters can be complicated to work with. For instance, as the physical filter fills up, filtration capacity is reduced. Also, using such filters incurs periodic stopping to remove the filter and/or obtain or clear the particles trapped thereon.
Acoustophoresis is the separation of materials using high intensity sound waves. High intensity standing waves of sound can exert forces on materials. An acoustic standing wave has a pressure profile with node and anti-node locations that are relatively stable over time. The pressure profile in an acoustic standing wave contains areas of net zero pressure at its nodes and anti-nodes. Depending on their density and compressibility, materials presented to the acoustic standing wave can be trapped at the nodes or anti-nodes of the standing wave.